As one of heat medium heating units provided in vehicle air conditioning apparatus applied to electric vehicles or hybrid cars or the like for heating a heat-transfer medium to serve as a heat source for heating operation, a Positive Temperature Coefficient (hereafter PTC element) heater is known that uses a PTC thermistor element as a heating element. The PTC heaters has a positive thermistor characteristic, in which the value of resistance thereof rises as the temperature rises, which controls the current consumption to make its temperature increase become mild, and in which, thereafter, the current consumption and the temperature of the heating portion reach a saturation level and stay unchanged. Thus, the PTC heater has a self-temperature control feature.
As a heat medium heating unit described above, PTL 1 discloses a heat medium heating unit having multiple partitions that separate the inside of the housing to make a heating chamber and a heat medium circulation chamber within the housing which has an inlet and an outlet for the heat medium, and the PTC heating element is inserted for installation in contact with the partition in the heating chamber defined by the partitions, and the heat medium circulated in the circulation chamber is heated over the partition by the PTC heating element.
Further, PTL 2 discloses a heat medium heating unit of a stacked structure in which an electrode plate, an insulating layer and a heat transfer layer are provided on both surfaces of a PTC element to constitute a plate-like PTC heater, and a pair of heat medium circulation boxes including an inlet and an outlet for a heat medium, and communicating with each other are stacked on both surfaces of the PTC heater, and a substrate housing box that houses a control board and a lid are provided on the external surface thereof.